FlareActor is not visible on web with master channel

[perclasson]

Steps to Reproduce

1. Create an app, from the following repo and branch example_for_testing_web_issue (is also reproducible with other examples, for example checkbox): https://github.com/perclasson/Flare-Flutter/tree/example_for_testing_web_issue/example/simple
2. Run the app with Flutter for Web with the Flutter master channel
3. The Flutter flare actor does not show up.

It works fine on the beta channel for web, and also master for iOS or any other platform. That is why I am creating this issue on flutter/flutter rather than the Flare repository.

Possible issue causing the regression

From running git bisect it seems like this issue was introduced in an the engine roll:
846dce9

From looking at the PRs submitted in the roll, I suspect that this is related to either:
flutter/engine#15153

Possibly also:
flutter/engine#15160

Flutter doctor

[✓] Flutter (Channel master, v1.14.4-pre.45, on Mac OS X 10.15.2 19C57, locale en-DE)
    • Flutter version 1.14.4-pre.45 at /Users/perc/development/flutter
    • Framework revision 92cbaa3efe (12 hours ago), 2020-01-24 05:43:01 +0100
    • Engine revision 439a21816e
    • Dart version 2.8.0 (build 2.8.0-dev.5.0 e84bea25df)

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Connected device (4 available)
    • Chrome            • chrome                               • web-javascript • Google Chrome 79.0.3945.130
    • Web Server        • web-server                           • web-javascript • Flutter Tools

Related issue

material-components/material-components-flutter-gallery#528

[jason-simmons]

Confirmed that the Flare example app works if flutter/engine@27a221d is reverted

@ferhatb

[ferhatb]

Update: we reproduced and fixed issue. Adding regression tests. Fix should land shortly.

[ferhatb]

Rolled into flutter master channel.